Bright Memory: Infinite

The way I did this (and other weapon kills achievements) is as follows:

Select the scene "Shadow Ghost" and play up until the point you call your vehicle. When you get to that point, go back to the main menu and select the same scene and do it over and over with each of the weapons. I Counted about 27 kills in about 2 mins each time making this quick and easy

If you have completed the game then it wont take as long to unlock the achievements

You can also collect 2 Reliquaries on this run as well, one when you start the level, turn around and one is sitting right behind you, the other will be on a platform when you jump down and see the shield users for the first time. Make sure you return to the main menu as reverting to the last checkpoint does not respawn them

(There is probably a quicker and easier way to do this but this is the way that worked for me for all weapons)

As the description says, just kill people using your handgun. Do it 50 times

This game has a lot of bugs. Just keep killing until it unlocks

It is cumulative. Can be done in multiple playthroughs